1. Field of the Invention
The present invention generally relates to optical scanners or readers, and more particularly to a device for reading retroreflective bar codes from a substrate such as a card.
2. Description of the Prior Art
Optical readers are used today in a wide variety of applications, such as optical character recognition, electronic facsimile transmission, and bar code scanning of Universal Product Codes which have been adopted by the retail industry. In order to understand the basic principles of optical scanning, it is necessary to distinguish between three types of reflections, namely, specular reflection, diffuse reflection, and retroreflection.
A familiar example of specular reflection is the image observed in a mirror or other highly polished surface. A beam of light striking such a surface narrowly reflects off the surface at an angle which is equal to the angle of incidence of the light beam. Diffuse reflection, in contrast, occurs when light strikes a dull or matte (Lambertian) surface, and distributes the light in all directions. Technically, diffuse reflection is actually a form of specular reflection in which the reflective material contains, on a microscopic level, a plurality of discrete surfaces having different orientations. This blurs the distinction between specular and diffuse components and, often, there is no true specular reflection, but rather a high-intensity lobe with a definite spread. For practical purposes, however, and for the purposes of discussion here, specular reflection is quite different from diffuse reflection, and most real surfaces exhibit both specular and diffuse reflection.
The third kind of reflection, retroreflection, occurs when incident light is narrowly reflected back along the incident beam path. This well-known phenomenon is usually achieved by the use of hemispherically-coated transparent microspheres embedded in a substrate; an example of such material is Scotchlite brand retroreflective sheeting, sold by Minnesota Mining & Manufacturing Company (3M), assignee of the present invention ("Scotchlite" is a trademark of 3M). See also U.S. Pat. No. 4,085,314 issued to Schultz et al. on Apr. 18, 1978.
Most optical scanners used today (including the laser scanners commonly seen in grocery stores) rely on diffuse reflection to read the coded markings. Specular reflections are too angle-dependent to be utilized in a practical manner. Retroflection, however, may advantageously be used in conjunction with scanning of optical marks, as exemplified in U.S. Pat. Nos. 3,225,177 (issued to Stites et al. on Dec. 21, 1965); 3,541,310 (issued to F. Stites on Apr. 17, 1970); and 3,569,676 (issued to Stites et al. on Mar. 9, 1971). Retroreflective coding can be scanned more rapidly and reliably, and may be read at greater distances, with a lower intensity light source, or with a less sensitive detector.
Unfortunately, no object exhibits perfect diffuse reflection, so some amount of specular reflection is unavoidable. These reflections often prove quite troublesome in scanning applications. For example, even a black mark can yield a specular reflection if the material forming the mark is sufficiently glossy, and such a reflection may actually be brighter than the diffuse reflection emanating from an adjacent white mark or the substrate. Specular reflections even affect retroreflective coding since they saturate the optical detector and diminish the effective contrast between the retroreflective mark and the plain substrate. This disadvantage is present in both "flying-spot" and "flying-aperture" type scanners The only prior art method used to compensate for this effect is to flood the mark with light and limit the collection angle, but this requires a much more intense light source and a complicated baffle system. It would, therefore, be desirable and advantageous to devise an improved optical reader which minimizes the adverse impact of specular reflections.